Question

I´m still having a hard time with Linq.
I need to write a Update Function tat receives an object that has a list. Actually, A region has a list of cities. I want to pass an object “Region” that has a name filed and a list of cities. The problem, is the city objects came from another context and I am unable to attach them to this context. I have been trying several functions, and always get an error like “EntitySet was modified during enumeration” or other. I am tring to make the code below work, but if anyone has a different approach please help.

    public int Updateregion(region E)
    {
        try
        {
            using (var ctx = new AppDataDataContext())
            {
                var R =
                       (from edt in ctx.regiaos
                        where edt.ID == E.ID
                        select edt).SingleOrDefault();              
                if (R != null)
                {
                    R.name = R.name;
                    R.description = E.description;
                }
                R.cities = null;
                R.cities.AddRange(Edited.Cities);
                ctx.SubmitChanges();
                return 0 //OK!
            }
        }
        catch (Exception  e)
        {
            ......
        }

Answer 1

You can’t attach objects retrieved from one datacontext to another, it’s not supported by Linq-to-SQL. You need to somehow dettach the objects from their original context, but this isn’t supported either. One can wonder why a dettach method isn’t available, but at least you can fake it by mapping the list to new objects:

var cities = Edited.Cities.Select(city => new City {
    ID = city.ID,
    Name = city.Name,
    /* etc */
});

The key here is to remember to map the primary key and NOT map any of the relation properties. They must be set to null. After this, you should be able to attach the new cities list, and have it work as expected.